LOS ANGELES — The Mars rover Opportunity is working on the biggest challenge it's faced since landing on the Red Planet last year: how to get out of a sand dune where it's been stuck for two weeks.

Engineers spent this week simulating the Martian terrain at NASA's Jet Propulsion Laboratory in Pasadena to try to figure out why the robot got bogged down and how to get it moving again. Engineers performed several tests driving a dummy rover over an artificial sand dune.

Scientists sent the first new driving directions to Opportunity on Wednesday, commanding it to position its wheels for a series of "mini-drives" out of the dune.

A status report on Friday said the rover team "proceeded with plans for Opportunity to rotate its wheels about two and a half times. ... Results from that move will be evaluated before rotating them some more."

The six-wheeled Opportunity had driven about 130 feet (40 meters) of a planned 295-foot (90-meter) trip when its wheels started to slip April 26. The rover, going backward at the time, eventually stopped moving — its wheels stuck hub deep in fine soil while trying to drive over a foot-high (30-centimeter-high) sand dune.

"Mars gives us surprises on a regular basis — some major, some minor," said project manager Jim Erickson. "This is something we consider a major one."

In the meantime, the rover has been taking pictures of its surroundings at the edge of an area known as the "etched terrain," where scientists believe they will uncover rocks exposed by the gentle erosion of wind.

Opportunity and its twin, Spirit, have been exploring opposite sides of Mars since landing in January 2004 and have uncovered geologic evidence of past water activity on the planet. Both rovers outlasted their primary, three-month missions long ago.

This is not the first time a rover has encountered a wheel problem. In June 2004, Spirit's right front wheel became balky. Spirit overcame the problem by driving in reverse, allowing it to drag the faulty wheel while driving itself with its five other wheels.